Oh yes! This fits 100%. It is perfectionism, and engagement. If the questions are too simple - he slows down even more. So on more complex questions/situations he speeds up. During his testing they noted it various times - he is engaged so there isn't any disability it is just the challenging factors. So no challenge = slower processing. For me it doesn't make sense, but I am not in his shoes, and he can't explain it.
This is not at all unusual. I wouldn't truly call it slow processing, I would call it poor fit of his educational environment to his learning needs for too long - resulting in unhealthy perfectionism.
My son - now 13 - described it as: The easy questions are so easy that I think they couldn't possibly be this easy, so I study them carefully to figure out what the trick is.
How sad!
Grinity
This might be the answer we are looking for. 
